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NEWSLETTER, WINTER 2010
PEMBROKESHIRE, APRIL 18-23, 2009
Staying in St Davids at the comfortable Old Cross Hotel in the city centre meant that we
didn’t have far to drive to our first site, Caerfai Bay (SM 759243), on the morning of Sunday
19th
April. In this part of the St Davids Peninsula about half of the Cambrian succession is
exposed, with beds dipping to the south on the southern limb of an anticline. From the cliff
top at Caerfai we looked over the 60-metre (200 ft) peneplain, which dominates the western
part of Pembrokeshire. It is thought to be Pliocene or early Pleistocene in age, but only head
and glacial deposits of the Last (Devensian) glaciation occur on it, so dating is difficult. The
green St Non’s Sandstone forms the landward margin of the bay, and seawards this is
followed by red shale and the red Caerbwdy Sandstone. Ripple marks indicating shallow
water during deposition of the Caerbwdy Sandstone are emphasized by grey, quartz-rich
sand. Within the bay three closely packed N-S faults have produced a fault breccia, and the
junction between the St Non’s and Caerbwdy Sandstones is also faulted.
We then moved on to Caerbwdy Bay (SM 769249). Descending to the beach, we admired
spring flowers of gorse, thrift, scurvy grass, violets and bird’s foot trefoil. Stone for St
Davids Cathedral was quarried on the western side of the bay from the Caerbwdy Sandstone.
On the beach we found a boulder of halleflinta, a fine silicified tuff, and on leaving the bay
we found an outcrop of the same rock near where we had parked. The rock here dips to the
north, on the northern limb of the anticline. The halleflinta is about 650 million years old and
is part of the Pebidian group of late Precambrian rocks.
After lunch in St Davids, we went on to Whitesands Bay (SM 734272), where the Cambrian
rocks are overlain unconformably by Lower Ordovician (Arenig) and intruded by the Carn
Llidi gabbro. The Middle Cambrian Solva and Menevian rocks seen towards the southern
margin of the bay are heavily cleaved, with the result that the bedding is difficult to
recognize, but various bedding structures at an acute angle to the cleavage suggest a N-S
strike. Peter demonstrated cleavage refraction resulting from textural variation in the
sediment and the greater spacing of cleavage planes in coarser beds. Middle Cambrian
dolerite forms an E-W dyke exposed in the middle part of the bay. On the northern side of
the bay we examined the Lingula Flags, but found no specimens of the brachiopod Lingula.
The Arenig rocks are exposed in the next bay to the north, from where we obtained a good
view to the west of St Davids Head, which is composed of Ordovician quartz gabbro. Carn
Llidi, also composed of quartz gabbro, is a large hill overlooking the northern side of
Whitesands Bay.
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On Monday 20th
April, we visited Abereiddi Bay (SM 797314), which exposes Lower and
Middle Ordovician graptolitic shales, and is famous as the type locality of the graptolite
Didymograptus murchisoni. Here we were in a syncline, with the cleavage parallel to the
bedding, commensurate with good preservation of fossils. Together with the dark colour of
the pyritic shales, this bode well for graptolite-hunting, but most of us were unlucky – except
Mervyn who found a nice specimen near the old quarry called the Blue Lagoon (SM
795315).
We next visited Mathry ((SM 890323), which is the site of an impressive kettle hole formed
in glacial deposits of the Devensian Stage (Last Glaciation). John explained how the kettle
hole formed by collapse of the deposits on the site of a buried block of glacial ice, which
slowly melted in situ. Because it is underlain by impermeable till, the hole is permanently
water-filled. The ice probably came from the Welsh Mountains, though the north coast of
Pembrokeshire was also invaded by Irish Sea ice from the Lake District and Scotland. Lunch
was then taken beside a misty Fishguard Harbour, though some walked into the town to visit
pubs and cafes. On the south side of the harbour we examined outcrops of the Fishguard
Volcanic Series, which consist of basic lavas and tuffs intruded by dolerite. Black shales
interbedded with the lavas help to date the volcanics as Ordovician, but again we found no
graptolites. The volcanics probably originated from an island arc within the Iapetus Ocean.
We then drove to Cwm Gwaun (SN 006349), which is one of a series of deeply incised
valleys north of the Preseli Hills. Previously interpreted as cut by overflow from glacial
lakes impounded between the Irish Sea ice and the hills, the valleys were later shown to have
humped (up-and-down) long profiles, and therefore probably originated by subglacial
meltwater under pressure, which can locally flow uphill. Strumble Head (SM 895412) was
wreathed in fog when we arrived. Near the car park, we visited a little quarry, where the
green mineral epidote could be seen on joint surfaces in basic pillow lavas of the Fishguard
Volcanic series. Below the nearby lookout building, the pillow lavas were better expressed.
On Tuesday April 21st, we drove to St Martin’s Haven (SM 762089) to examine the coarse
porphyritic and finer basalts of the Skomer Volcanic Series on the beach. These are Upper
Llandovery (Lower Silurian) in age. The associated sediments are mainly coarse sandstones
and quartzites deposited near the shore of the closing Iapetus Ocean, in which the Ordovician
shales seen on Monday had been deposited in somewhat deeper water, when the ocean was
wider. At Marloes Sands on the southern side of the Marloes Peninsula, we saw younger
Silurian (uppermost Llandovery to Wenlock) sediments, including limestones crowded with
crinoid ossicles, corals, brachiopods, bryozoans and trace fossils (burrows). The near vertical
dip of the beds here results from the Armorican earth movements. Tim led us to a very
fossiliferous block of downfaulted Wenlock deposits.
After lunch at the pub in Dale, we drove to St Ann’s Head (SM 805030), where we had
wonderful views of the Old Red Sandstone, which forms the south-eastern edge of Marloes
Bay. In Cobblers Hole, we admired a large concentric fold with an E-W axis. Tim reported
that a shale bed nearby contains Devonian spores. At Musselwick (SM 851128), we stopped
to view a cliff section, which exposed Precambrian rocks thrust over Coal Measures during
the Amorican orogeny.
On Wednesday April 22nd
, we drove along the coast of St Brides Bay to Little Haven (SM
857128), where the cliffs expose sandstones, shales and coal seams of the Coal Measures,
again folded in the Armorican orogeny (Fig. 1). The folds are well displayed in the cliffs.
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Fig. 1. Coal seams at Little Haven
Fig. 2. Gash breccia in Carboniferous Limestone at Trevallen
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Other folds seen between Broad Haven (SM 862140) and Little Haven constitute typical thin-
skin tectonics, with folds riding over others piggy-back style. After lunch in Pembroke, we
drove to Freshwater East (SS 016977), where Silurian rocks are exposed beneath the Old Red
Sandstone in a major E-W Armorican anticline. The southern margin of the bay is formed of
Ludlow beds containing Rhynchonellid brachiopods and gastropods. These are overlain by
grey and red sandstones and conglomerates of the Old Red Sandstone.
At Manorbier (SS 064977), we walked to the beach past the imposing castle. As before, in
the shore exposures we could pick out the near-vertical bedding in the Old Red Sandstone by
green marl beds, whose colour resulted from reduction of iron in the presence of organic
matter in local ponds and lakes. The bedding was also picked out crudely by pink calcium
carbonate nodules, which were formed by rapid evaporation in the arid desert environment of
the Old Red Sandstone.
Thursday April 23rd
was devoted to the study of gash breccias in south Pembrokeshire in
the company of Peter Walsh and Sid Howells. These enigmatic deposits consist of large
jumbled angular blocks of Carboniferous Limestone filling huge vertical pipe-like
depressions in the in situ Carboniferous Limestone of the Bullslaughter Bay syncline. We
met the leaders at the National Trust’s Broadhaven car park (SR 976938) and walked to the
first site, close to the Trevallen army firing range (Fig. 2). Here Peter outlined the numerous
theories put forward in the past to explain the breccias. He favoured an origin as Cretaceous
hydrothermal diatremes, that is formed when steam under pressure forced its way to the
surface from considerable depth, thus creating a cylindrical pipe of shattered rock. There are
11 other examples all within a relatively small area, but it isn’t clear why they do not occur in
the Carboniferous Limestone elsewhere. The Trevallen gash has a total volume of at least
200,000 cubic metres, though the depth is unknown. The gashes clearly predate the 60 m
peneplain of Pembrokeshire, but this doesn’t help much with their precise dating. They often
contain red sediment, probably Triassic, and are often cemented with secondary carbonate.
After lunch, we travelled to Lydstep Haven, where the Carboniferous Limestone has a dip of
110o, i.e. has been overturned beyond the vertical by Variscan earth movements. Lydstep
Point (SS 095976) to the south exposes another gash breccia. From the cliff top here, we
looked across using binoculars to Valley Field Top and St Margarets Island (SS 119973),
where there are two more gash breccias. The last of these is 200 m across, 50 m wide and at
least 30 m deep, though the northern and western margins are concealed beneath the sea; the
total volume is at least 250,000 cubic metres. We then went on to Lydstep Cleft, to examine
a vertical karstic depression in limestone filled with reddened material, which raised yet more
apparently unanswerable questions!
On Friday April 24th
, the group split up, individuals choosing their own sites to visit,
including a boat tour of Ramsay Island to look at seabirds, seals and the cliff exposures of
Ordovician sediments, quartz porphyry and gabbro. Whatever was done, everyone seemed
very happy at the end of the day with their respective choices. It was a fitting end to a lovely
week, blessed with good weather, interesting geology and a welcoming hotel. Our thanks to
Peter Banham, Peter Walsh, Sid Howells and John Catt for organizing and leading the week.
It was a pity illness prevented another leader, Barrie Rickards from Cambridge, from joining
us.
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BERKHAMSTED, MAY 9, 2009
By John Catt
For this excursion we were joined by members from several other societies, including some
who had travelled that morning from Suffolk, Bedfordshire and the East Midlands. Initially
we met to park cars on the bridge where Swing Gate Lane crosses the new A41 on the
southern side of Berkhamsted (SP 997065). From this point we walked down the lane to
Bottom Farm on the floor of the Hertfordshire Bourne valley, where we were met by John
Marsland, the present owner of Bottom Farm.
The Hertfordshire Bourne was made famous by John (later Sir John) Evans, who was the first
in 1878 to describe its intermittent flow. Later, whenever the bourne flowed, it was visited
by members of the Hertfordshire Natural History Society, often under the leadership of John
Hopkinson. An early myth was that the bourne flowed every seven years, but the records by
Evans, Hopkinson and others indicated more frequent flow at less regular intervals.
Whenever rainfall in the previous calendar year exceeded 32 inches, flow occurred for a
period between January and June. However, various parts of the valley were occupied by the
surface stream in different years or even the same year. Very often flow starts at a series of
springs just above White Hill (SP 991052), and sometimes a small lake accumulates here
beside the road, but at other times it has started a short distance downstream below Mounts
Hill. Sometimes the flow terminates in a swallow hole just upstream from Bottom Farm, but
more often it flows through the garden of Bottom Farm and across the meadow to the east
(Fig. 3), then terminating either in a large gravel pit (TL 005061), which acts as another
swallow hole or, when this overflows, the stream can extend through a culvert under the new
A41 to the appropriately named hamlet of Bourne End, where the bourne joins the River
Bulbourne.
On our visit, the first swallow hole near Bottom Farm was almost full of stagnant water, but
flow into it had ceased. John Marsland showed photographs of the bourne flowing through
his garden in early 2001 and 2007, and also made others available on his website:
http://picasaweb.google.com/john.marsland. We then walked through the very pleasant
flower meadow below the farm to the gravel pit, which still contained about 0.5 m depth of
water. The dry course of the bourne could be traced across the meadow into the pit, but not
in the arable field beyond the pit. The water levels in the swallow hole and gravel pit
therefore indicated the temporary level of the water table beneath the valley floor, and flow
over the surface occurs only when the water table rises above the level of the channel in any
stretch of the valley.
After returning to the cars, we dispersed for lunch at various pubs in the Berkhamsted area,
and then re-assembled in Castle Hill, Berkhamsted (SP 993085) to examine several large
blocks of Hertfordshire Puddingstone on the grass verge outside the house (No. 13) of
Professor Eric Brown. The largest block is partly sarsenstone (silcreted sandstone usually
devoid of flint pebbles), though the sarsen part contained a line of dispersed pebbles. The
importance of this specimen, demonstrating the close relationship between puddingstone and
sarsen, has been recognized in designation of the group as a Hertfordshire RIGS site. The
leader explained that puddingstone and sarsen originated by silicification of pebble beds or
sand in either the Upnor Formation (late Palaeocene) or Reading Formation (early Eocene).
As these deposits extended up the sub-Palaeogene erosion surface now exposed as the
http://picasaweb.google.com/john
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dipslope of the Chilterns, the blocks had moved down the slope of Castle Hill from the
plateau surface north of the Bourne Valley. The silcretes had probably formed within soil
profiles developed in humid subtropical conditions on these formations in the Palaeogene, i.e.
soon after their deposition.
The final site of the afternoon was the small gravel pit at Little Heath (TL 017082). This
exposes about 2 m of red sand over a coarse gravel composed of rounded flint cobbles. The
sand had originally been dug during World War I to fill sand bags for protection of London’s
houses from aerial bombing. Only 0.3 m of the topmost gravel could be seen, but the leader
explained that re-excavation of the pit in 1978 had exposed over 6 m of gravel, resting on
thin representatives of the Reading and Upnor Formations. Chalk had been proved by
augering about 10 m below the ground surface, which is here approximately 160 m OD. In
addition to flint, the gravel contains numerous small white quartz pebbles and occasional
cobbles of Hertfordshire Puddingstone, the latter suggesting that the gravel is younger than
the Reading Formation. The source of the quartz pebbles is unknown, though it has been
suggested that they are derived from the Lower Greensand west of the Chalk scarp.
As the deposits occur no more than 15 m above the projected level of the highest proto-
Thames terrace gravels on the Chiltern dipslope, they could be only a little older than this
terrace. However, the leader explained that the gravel is marine in origin, because it contains
glauconite and the coarse but well-sorted nature of the gravel suggests deposition on a sea
beach. At present the deposit is thought to be late Pliocene or earliest Pleistocene in age
(about 2.6 Ma). The most likely equivalent is the Red Crag, which is known to extend inland
from the North Sea coast in Suffolk and Essex, rising in height from near OD on the coast to
about 90 m OD at Stansted Mountfitchet (near the Essex-Hertfordshire border) and 130 m
OD at Rothamsted near Harpenden, Hertfordshire.
DUNSTABLE DOWNS, JUNE 14, 2009
By John Catt
This excursion followed a conference on the Chalk of Hertfordshire, held at Verulamium
Museum on Saturday June 13, and was led by the conference speakers: Drs Haydon Bailey,
Trevor James, Stuart Bryant, Isobel Thompson and John Catt. After assembling at the
National Trust car park (TL 008198) at 11 am, we walked through the woods to an old part of
Kensworth Quarry (TL 011199) where the Bedfordshire and Luton Geological Society have
preserved a section across the boundary between the Middle and Upper Chalk. The section
exposes the very fossiliferous Chalk Rock and a marl band (the Caburn Marl) in the upper
part of the Middle Chalk beneath. The quarry owners (Rugby Portland Cement) had kindly
provided nearby a large dump of Chalk Rock from another part of the working quarry,
allowing people to collect fossils without damaging the preserved face. Trevor James
pointed out how the vegetation growing on the preserved face is becoming concentrated
along the marl band. John Catt explained that Plateau Drift derived from a thin cover of
Reading or Upnor Formation sediments is preserved in solution pipes and cavities extending
down from the sub-Palaeogene erosion surface, and Trevor suggested that the non-calcareous
pipe deposits account for dispersed specimens of oak trees growing in younger parts of the
woodland on slopes above the preserved section.
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Fig. 3. The Hertfordshire Bourne looking up valley towards Bottom Farm
Fig. 4. The Five Knolls, Dunstable Downs
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After lunch at the NT Chiltern Gateway Centre, Haydon used a bronze model outside the
centre to demonstrate the influence of Lower Cretaceous bedrock geology on the landscape to
the northwest as seen from the crest of the Chalk escarpment. The group then walked north-
eastwards along the scarp crest towards Dunstable to view geomorphological and
archaeological features. John Catt outlined the various theories put forward to account for
the steep scarp-slope dry valleys, and drew attention to the small wooded Well Head valley
(SP 999204) at the scarp foot close to the Dunstable Gliding Club’s airfield. This valley has
been formed by the process of spring sapping, which may partly account also for the larger
scarp slope valleys. The springs rising in the valley are a source of the River Ouzel, which
flows through the town of Leighton Buzzard. Stuart Bryant and Isobel Thompson spoke
about evidence for the prehistoric occupation of the Downs, and showed us the group of late
Neolithic or Bronze Age round burial mounds known as the Five Knolls (TL 006210) (Fig.
4). From this location, we were also able to view Totternhoe Knoll (SP 979221), the site of a
Norman Motte and Bailey castle, and the Iron Age hillfort site of Maiden Bower (SP
997224). Both occur on outliers of the Melbourne Rock at the base of the Middle Chalk. An
extensive quarry at a lower level to the northwest was the source of Totternhoe Stone, a
freestone occurring within the Lower Chalk, which has been quarried probably since Roman
times. The stone was used for carved window and door frames in many Hertfordshire
churches, and for this reason the quarries had been owned by St Albans Abbey in the later
Middle Ages.
JOINT MEETING WITH EAST MIDLANDS GEOLOGICAL SOCIETY, JULY 11/12
By John Catt
In preparation for two days fieldwork in Hertfordshire and Bedfordshire, short talks were
given to members of both societies at Verulamium Museum by Haydon Bailey, Peter
Banham and John Catt on the evening of Friday July 10. The following morning, we
assembled again at Dunstable Downs to visit the Kensworth section, and then visited Little
Heath before lunch. After lunch we revisited the puddingstone/sarsenstone blocks in Castle
Hill, Berkhamsted and walked along part of the Hertfordshire Bourne valley. All these sites
are described above.
On Sunday July 12, we assembled at 10 am at the National Trust Pitstone Hill car park (SP
955150), and walked across the meadows to the edge of Steps Coombe or Incombe Hole (SP
959156), a spectacular Chiltern scarp face dry valley near Ivinghoe (Fig. 5). Here John Catt
again outlined the various theories advanced to explain such coombes, namely erosion by
meltwater from snow and ice on the Chiltern plateau surface, spring sapping, late-glacial
gelifluction and nivation. He suggested that they could best be explained by a combination
of spring sapping and gelifluction. Like the Ravensburgh and Barton valleys in northeast
Hertfordshire, the course of Steps Coombe shows right-angle bends, which have been
attributed to structural control of erosion by the pattern of joints in the Chalk. However, Eric
Brown (1969) found no correspondence between the directions of the various segments of
Steps Coombe and the joint patterns in nearby Chalk quarries, so the exact mode of origin
and development of the valley and others like it remains obscure.
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Fig. 5. Incombe Hole near Ivinghoe
Fig. 6. View of Denbies Vineyard near Dorking
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We then drove to the car park beside Wilstone Reservoir (SP 904135), and walked round the
western side of the reservoir to visit the dry course of the Wendover Arm of the Grand Union
Canal. On the way Peter Banham pointed out several springs that feed the reservoir and rise
from the Totternhoe Stone and other hard fractured layers within the Grey (Lower) Chalk.
One spring (at SP 908127) occurs in the floor of a deep narrow valley that must have
originated by the process of spring sapping. Peter outlined the history of the Wendover
Arm, intended as a feeder for the main Grand Union Canal, and explained that it had leaked
because it crossed the axis of a minor syncline, so that a section of it was cut in the fissured
upper part of the Lower Chalk instead of the much less permeable Chalk Marl. After walking
along a short section of the canal to view the results of attempts to seal and reinstate it, we
returned to the cars and went for lunch at Startop’s End.
After lunch we drove to Stone Lane Quarry at Heath and Reach (SP 927290) to see an
abandoned and graded section in the Woburn Sands overlain by Gault Clay and Chalky
Boulder Clay. In front of the explanatory board erected at the entrance to the quarry by the
Bedfordshire and Luton Geological Society, Peter demonstrated the various parts of the
Woburn Sands previously exposed here, and explained the problems of conservation of the
numerous geological sections in the Leighton Buzzard area.
For the final site of the day, we drove to Husborne Crawley (SP 955362), to examine a bright
green, partially silicified sandstone in the walls and tower of the church. The sandstone is
thought to have been quarried on the northern side of the steep-sided mound on which the
church is built. This site is close to the base of the Woburn Sands, where they overlie
Ampthill and Oxford Clay. John Catt said he had found blocks of a similar bright green
sandstone and uncemented green glauconitic sand near the base of the Lower Greensand
while mapping the soils on part of Woburn Experimental Farm a short distance to the east of
the church. He thought that the green colour of unoxidized glauconite had been preserved
where the basal sand was waterlogged just above the clay. This situation could also explain
redeposition of silica from water percolating through the sand to form the sandstone.
Through the kindness of the churchwardens, we were able to enter the church and Mike
Howgate explained how weak foundations had caused the church walls to crack so badly that
much of it had to be rebuilt in the early 20th
century. Stages in the rebuilding were illustrated
by early photographs in a small exhibition inside the church.
Dr Ian Sutton proposed a vote of thanks to all the leaders of the weekend meeting, and from
Husborne Crawley our visitors from the East Midlands left for home by way of the nearby
M1.
VISIT TO DORKING AND DENBIES VINEYARD, SEPTEMBER 5
By Linda Hamling
We met our leader, Professor Dick Selley of Imperial College London and self-styled
notorious local geologist, in the car park of Denbies Wine Estate, which lies in the lee of Box
Hill. From here, we drove into Dorking to visit Dorking Caves, the entrance to which is well
hidden behind an obscure steel door at the side of the town’s war memorial. They are dug in
Folkestone Sands, which are soft and easily excavated, yet strong enough to support tunnels
without collapsing. In the cave walls, we could see prominent cross-bedding, indicating
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deposition in a shallow sea. As part of the Lower Greensand, the sands were originally
glauconitic and green, but they have been oxidized, releasing iron from the glauconite, which
has been reprecipitated to form hardened concretions and layers (carstone) and create
Leisegang rings in the sand. The caves are probably late mediaeval, and consist of well
shafts leading to tunnels dating from the 17th
and 19th
centuries. Previously they were used as
wine vaults, and some of the tunnels allow access to stalls, which are still labelled with their
former contents. Dick showed us some old graffiti before we descended the 20-metre
staircase leading to a mystery chamber, which is a circular cave with a carved bench around
its wall. The purpose of the chamber is unknown. Chalybeate water rich in iron must have
flooded the cave at one time, leaving a ‘tidemark’ around the wall. We left the caves to
emerge back into the bustle of the town, and collected the cars to drive up to Ranmore
Common, where we sat in the sunshine to eat our lunch. From this picnic spot, we had a
view below of the Mole Valley and the Vale of Holmesdale along the outcrop of the Gault
Clay, with the northward-dipping Lower Greensand forming Leith Hill on the other side of
the valley.
The Dorking area has been used for vine growing for many centuries. Mediaeval vineyards
were abandoned early in the Little Ice Age, but in the 1600s the Duke of Norfolk re-
established one on the Chalk at Albury Park, and another was planted at Painshill on Bagshot
Beds in the 1700s. After lunch, we walked past St Barnabas Church, Ranmore, which was
designed by Gilbert Scott for the Cubitt family, who owned the Denbies Estate. Now the
estate is owned by Adrian White of Biwater Water Treatment. From a track descending the
Chalk scarp slope, we looked through a gap in the hedge across the vineyards in a dry valley
towards Box Hill. Dick had been responsible for suggesting to Adrian White that the south-
facing valley and lower scarp slope of the North Downs would be an ideal site for growing
vines, as they had soils similar to those of the Champagne region of France, and offered
maximum sunshine and protection from cold winds. Most of the present vineyard is planted
at lower levels on a raised Pleistocene terrace of the River Mole (Fig. 6). As we walked
down the hillside, we admired harebells, thyme, scabious, knapweed and St Johns wort on the
banks of the track, and looked down on a house built entirely of Lego for a television
programme by James May.
On reaching the main estate buildings again, we enjoyed an Imax multiscreen film about
establishment of the vineyard, which featured Dick, and then had a short tour by train of the
wine-making process. This was followed by a tutored tasting of several different wines, and
the opportunity to buy the wide range of wines in the excellent vineyard shop. Our cars were
distinctly heavier for the return to Hertfordshire! We are especially grateful to Dick for a
wonderful and convivial day and for introducing us to an unusual and important aspect of
applied geology.
VISIT TO BRIGHTON, OCTOBER 10
By John Catt
Following Colin Whiteman’s talk on October 8th
, four of us visited Brighton to see the
magnificent interglacial buried cliff and raised beach section at Black Rock (TQ 336033).
On a brilliant sunny autumn morning, we assembled on the top floor of the multistory car
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park at Brighton Marina. From this viewpoint, we could take in the whole cliff section
showing the relationship of the buried cliff to the raised beach and overlying chalky slope
deposits on the eastern side of the Sheepcote dry valley. Recent attention had been focused
on the site following a cliff fall that had damaged part of the supermarket. Although the
original intention was to cover the whole of the high cliff in concrete, Natural England and
local geologists had persuaded Brighton Council to preserve this important exposure for
visitors like ourselves to view. Part of the section has been stabilized with hundreds of rock
bolts, and rock falls from another part are prevented by a cover of wire netting.
We then walked through the supermarket car park to get a closer view of the deposits
associated with the buried cliff. The raised beach deposit, which rests on a wave-cut platform
at about 7 m OD, consists mainly of rounded flint pebbles, but Colin had also found over 200
erratics in it, mainly greywackes and pink granites. He suggested that most came from NW
France, and had been carried eastwards up the English Channel by longshore drift. A few
bones and mollusc shells had also been found in the beach deposit, including bones of a small
horse (Equus ferus), which became extinct about 200,000 years ago at the end of the
penultimate interglacial (Marine Isotope Stage 7). This means the beach cannot be Last
Interglacial (MIS 5e), as previously thought. It was probably cut when the sea level was 15
m below present, and its present height can be attributed to tectonic uplift. The relatively
high level of the beach has probably resulted from tectonic uplift. Closer to the buried cliff,
the beach shingle is interbedded with masses of large chalk boulders, which probably
represent falls from the almost vertical interglacial chalk cliff.
Colin explained that the very thick chalky slope deposits that bury the beach and cliff were
deposited mainly by fluvial erosion processes on the slopes of the dry valley rather than mass
movement by gelifluction as previously thought. They contain a few cold-tolerant terrestrial
molluscs. Recent optically stimulated thermoluminescence dates on sandy layers within the
slope deposits indicate a late MIS 7 age for most it, so the underlying beach could be early
MIS 7 or older. Further west there is a sequence of buried cliffs and raised beaches at
increasing heights, which have been dated to earlier interglacials (MISs 9, 11 and 13).
Walking east, we examined the nature of the Newhaven Chalk, which forms the high chalk
cliffs for several miles east of Brighton. It contains lines of flint nodules and also thin marl
seams, which probably originated by weathering of volcanic ashfalls into the Chalk sea.
There are numerous horizontal and steeply inclined tabular or sheet flints, which result from
mobilization of the silica of flint nodules and its redeposition in fissures created by joints and
fractures in the Chalk. Periglacial disturbance of the uppermost few metres of the cliff during
the Devensian had deformed one of the horizontal tabular flints, and broken it into small
angular fragments, forming a gravel disturbed by flask-shaped involutions penetrating
downwards into frost-shattered upper layers of the Chalk.
After lunch, we walked on to Ovingdean, where we all squeezed into Colin’s car, and were
driven to Newhaven to look at the coastal section (TQ 447001) of basal Palaeogene (Upnor
and Reading Formation) sands and gravels overlying Newhaven Chalk Formation. The
section here showed iron-pan layers in the sands and gravels, fragments of which had been
observed in the raised beach gravel and overlying slope deposits at Black Rock, where they
are probably derived from small outliers of Reading Beds on the South Downs close to
Brighton. Higher in the Newhaven exposure were compacted peaty layers with tree roots and
clays containing crystals of gypsum, which had formed by oxidation of pyrite in the clay to
form sulphuric acid, and then reaction of the acid with calcium carbonate.
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